What does ?: do in Kotlin? (Elvis Operator)

Question

I can\'t figure out what ?: does in for example this case

val list = mutableList ?: mutableListOf() 

and why can it be modifi

Answer 1

Let's take a look at the defintion:

When we have a nullable reference r, we can say "if r is not null, use it, otherwise use some non-null value x":

The ?: (Elvis) operator avoids verbosity and makes your code really concise.

For example, a lot of collection extension functions return null as fallback.

listOf(1, 2, 3).firstOrNull { it == 4 } ?: throw IllegalStateException("Ups")

?: gives you a way to handle the fallback case elgantely even if you have multiple layers of fallback. If so, you can simply chain multiply Elvis operators, like here:

val l = listOf(1, 2, 3)

val x = l.firstOrNull { it == 4 } ?: l.firstOrNull { it == 5 } ?: throw IllegalStateException("Ups")

If you would express the same with if else it would be a lot more code which is harder to read.